Rabbeinu Yonah in Bava Basra 39a talks about someone who does Teshuva and calls him a Ben Teshuva. What is the earliest reference in history to the words Baal Teshuva? And why did it change from Ben Teshuva to Baal Teshuva?

But Rambam's code played a very strong influence on a lot of how we name things, so it doesn't surprise me that it won out. (Another interesting one is the phrase "Korban Pesach" ["Passover sacrifice."]) In the Talmud it's just "the Pesach" and you have to realize it means the sacrifice. It appears a small handful of times in various less-authoritative midrashim; then Rambam started using it in his Commentary on the Mishnah; then codified Laws of the Korban Pesach (vs. Laws of Chametz & Matza), and the term's popularity exploded ever since.